Reduced capacitive baseline shift via mixing period adjustments

ABSTRACT

A method and related processing system and input device are disclosed, the method comprising driving a first capacitive sensing signal with first sensing frequency onto a first group of a plurality of sensor electrodes, and acquiring first capacitive measurements of resulting signals received by a second group of the plurality of sensor electrodes. Acquiring first capacitive measurements comprises applying a first demodulation signal with a predefined first mixing period defined within a sensing period associated with the first sensing frequency. The method further comprises driving a second capacitive sensing signal having a second sensing frequency different than the first sensing frequency onto a third group of the plurality of sensor electrodes, and acquiring second capacitive measurements of resulting signals received by a fourth group of the plurality of sensor electrodes. Acquiring second capacitive measurements comprises applying a second demodulation signal having a different predefined second mixing period.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 15/400,681, filed Jan. 6, 2017, entitled “REDUCED CAPACITIVEBASELINE SHIFT VIA MIXING PERIOD ADJUSTMENTS”, which is hereinincorporated by reference in its entirety.

BACKGROUND Field

Embodiments disclosed herein generally relate to electronic devices, andmore specifically, techniques for reducing a capacitive baseline shiftbetween different sensing frequencies through selectable mixing periods.

Description of the Related Art

Input devices including proximity sensor devices (also commonly calledtouchpads or touch sensor devices) are widely used in a variety ofelectronic systems. A proximity sensor device typically includes asensing region, often demarked by a surface, in which the proximitysensor device determines the presence, location and/or motion of one ormore input objects. Proximity sensor devices may be used to provideinterfaces for the electronic system. For example, proximity sensordevices are often used as input devices for larger computing systems(such as opaque touchpads integrated in, or peripheral to, notebook ordesktop computers). Proximity sensor devices are also often used insmaller computing systems (such as touch screens integrated in cellularphones).

SUMMARY

One embodiment described herein is a method comprising driving, onto afirst group of a plurality of sensor electrodes, a first capacitivesensing signal having a predefined first sensing frequency, andacquiring, based on the driven first capacitive sensing signal, firstcapacitive measurements of resulting signals received by a second groupof the plurality of sensor electrodes, wherein acquiring firstcapacitive measurements comprises applying a first demodulation signalhaving a predefined first mixing period defined within a sensing periodassociated with the first sensing frequency. The method furthercomprises driving, onto a third group of the plurality of sensorelectrodes, a second capacitive sensing signal having a second sensingfrequency different than the first sensing frequency, and acquiring,based on the driven second capacitive sensing signal, second capacitivemeasurements of resulting signals received by a fourth group of theplurality of sensor electrodes, wherein acquiring second capacitivemeasurements comprises applying a second demodulation signal having apredefined second mixing period within a sensing period associated withthe second sensing frequency, the second mixing period different thanthe first mixing period.

Another embodiment described herein is a processing system comprising asensing module comprising sensing circuitry and configured to drive,onto a first group of a plurality of sensor electrodes, a firstcapacitive sensing signal having a predefined first sensing frequency,and acquire, based on the driven first capacitive sensing signal, firstcapacitive measurements of resulting signals received by a second groupof the plurality of sensor electrodes, wherein acquiring firstcapacitive measurements comprises applying a first demodulation signalhaving a predefined first mixing period defined within a sensing periodassociated with the first sensing frequency. The sensing module isfurther configured to drive, onto a third group of the plurality ofsensor electrodes, a second capacitive sensing signal having a secondsensing frequency different than the first sensing frequency, acquire,based on the driven second capacitive sensing signal, second capacitivemeasurements of resulting signals received by a fourth group of theplurality of sensor electrodes, wherein acquiring second capacitivemeasurements comprises applying a second demodulation signal having apredefined second mixing period within a sensing period associated withthe second sensing frequency, the second mixing period different thanthe first mixing period.

Another embodiment described herein is input device comprising aplurality of sensor electrodes and a processing system coupled with theplurality of sensor electrodes. The processing system is configured todrive, onto a first group of a plurality of sensor electrodes, a firstcapacitive sensing signal having a predefined first sensing frequencyand acquire, based on the driven first capacitive sensing signal, firstcapacitive measurements of resulting signals received by a second groupof the plurality of sensor electrodes, wherein acquiring firstcapacitive measurements comprises applying a first demodulation signalhaving a predefined first mixing period defined within a sensing periodassociated with the first sensing frequency. The processing system isfurther configured to drive, onto a third group of the plurality ofsensor electrodes, a second capacitive sensing signal having a secondsensing frequency different than the first sensing frequency, andacquire, based on the driven second capacitive sensing signal, secondcapacitive measurements of resulting signals received by a fourth groupof the plurality of sensor electrodes, wherein acquiring secondcapacitive measurements comprises applying a second demodulation signalhaving a predefined second mixing period within a sensing periodassociated with the second sensing frequency, the second mixing perioddifferent than the first mixing period.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentdisclosure can be understood in detail, a more particular description ofthe disclosure, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlyexemplary embodiments and are therefore not to be considered limiting ofits scope, may admit to other equally effective embodiments.

FIG. 1 is a schematic block diagram of an input device, according toembodiments described herein.

FIGS. 2 and 3 illustrate portions of exemplary sensor electrodeimplementations, according to embodiments described herein.

FIG. 4 illustrates an exemplary processing system for applying selectedmixing periods for different sensing frequencies, according toembodiments described herein.

FIG. 5 is a schematic diagram of a sensing implementation for applyingselected mixing periods for different sensing frequencies, according toembodiments described herein.

FIG. 6 illustrates a method for applying selected mixing periods fordifferent sensing frequencies, according to embodiments describedherein.

FIG. 7 is a diagram illustrating exemplary operation of a sensingimplementation without performing adjustments of the mixing period,according to embodiments described herein.

FIG. 8 is a diagram illustrating exemplary operation of a sensingimplementation with adjustments performed to the mixing period,according to embodiments described herein.

FIG. 9 is a schematic diagram of a sensing implementation for operatingone or more switching elements at different sensing frequencies,according to embodiments described herein.

FIG. 10 illustrates a method for operating one or more switchingelements at different sensing frequencies, according to embodimentsdescribed herein.

FIG. 11 is a diagram illustrating exemplary operation of a sensingimplementation using one or more switching elements, according toembodiments described herein.

FIG. 12 is a graph illustrating changes in capacitance across differentsensing frequencies, according to embodiments described herein.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures. It is contemplated that elements disclosed in oneembodiment may be beneficially utilized on other embodiments withoutspecific recitation. The drawings referred to here should not beunderstood as being drawn to scale unless specifically noted. Also, thedrawings are often simplified and details or components omitted forclarity of presentation and explanation. The drawings and discussionserve to explain principles discussed below, where like designationsdenote like elements.

DETAILED DESCRIPTION

The following detailed description is merely exemplary in nature and isnot intended to limit the disclosure or the application and uses of thedisclosure. Furthermore, there is no intention to be bound by anyexpressed or implied theory presented in the preceding background,summary, or the following detailed description.

Various embodiments of the present technology provide input devices andmethods for improving usability. An input device may include electrodesthat are operated as sensor electrodes to detect interaction between theinput device and an input object (e.g., a stylus or a user's finger).

An input device generally drives a capacitive sensing signal onto sensorelectrodes to acquire capacitive measurements. The sensing frequencycorresponding to the capacitive sensing signal may be adaptively changedby the input device, e.g., based on detected interference. However,capacitive measurements acquired at different sensing frequenciesrequire some compensation, as the different sensing frequenciesgenerally correspond to different capacitive baselines for thecapacitive measurements.

In some embodiments, a plurality of mixing periods for a demodulationsignal is selected corresponding to a plurality of predefined sensingfrequencies. In some cases, the mixing periods are selected such that aplurality of average current values corresponding to a plurality ofacquired baseline capacitive measurements have a predefined relationshipwith the different sensing frequencies. Thus, for any transitionsbetween different ones of the predefined sensing frequencies duringoperation of the input device, the corresponding average current valueswill have a predictable relationship, which simplifies any correction orcompensation that may be needed for the acquired capacitive sensingmeasurements.

Exemplary Input Device Implementations

FIG. 1 is a schematic block diagram of an input device 100, inaccordance with embodiments of the present technology. In variousembodiments, input device 100 comprises a display device integrated witha sensing device. The input device 100 may be configured to provideinput to an electronic system 150. As used in this document, the term“electronic system” (or “electronic device”) broadly refers to anysystem capable of electronically processing information. Somenon-limiting examples of electronic systems include personal computersof all sizes and shapes, such as desktop computers, laptop computers,netbook computers, tablets, web browsers, e-book readers, and personaldigital assistants (PDAs). Additional example electronic systems includecomposite input devices, such as physical keyboards that include inputdevice 100 and separate joysticks or key switches. Further exampleelectronic systems include peripherals such as data input devices(including remote controls and mice), and data output devices (includingdisplay screens and printers). Other examples include remote terminals,kiosks, and video game machines (e.g., video game consoles, portablegaming devices, and the like). Other examples include communicationdevices (including cellular phones, such as smart phones), and mediadevices (including recorders, editors, and players such as televisions,set-top boxes, music players, digital photo frames, and digitalcameras). Additionally, the electronic system could be a host or a slaveto the input device.

The input device 100 can be implemented as a physical part of theelectronic system, or can be physically separate from the electronicsystem. As appropriate, the input device 100 may communicate with partsof the electronic system using any one or more of the following: buses,networks, and other wired or wireless interconnections. Examples includeI²C, SPI, PS/2, Universal Serial Bus (USB), Bluetooth, RF, and IRDA.

In FIG. 1, the input device 100 is shown as a proximity sensor device(also often referred to as a “touchpad” or a “touch sensor device”)configured to sense input provided by one or more input objects 140 in asensing region 170. Example input objects include fingers and styli, asshown in FIG. 1.

Sensing region 170 encompasses any space above, around, in and/or nearthe input device 100 in which the input device 100 is able to detectuser input (e.g., user input provided by one or more input objects 140).The sizes, shapes, and locations of particular sensing regions may varywidely from embodiment to embodiment. In some embodiments, the sensingregion 170 extends from a surface of the input device 100 in one or moredirections into space until signal-to-noise ratios prevent sufficientlyaccurate object detection. The distance to which this sensing region 170extends in a particular direction, in various embodiments, may be on theorder of less than a millimeter, millimeters, centimeters, or more, andmay vary significantly with the type of sensing technology used and theaccuracy desired. Thus, some embodiments sense input that comprises nocontact with any surfaces of the input device 100, contact with an inputsurface (e.g. a touch surface) of the input device 100, contact with aninput surface of the input device 100 coupled with some amount ofapplied force or pressure, and/or a combination thereof. In variousembodiments, input surfaces may be provided by surfaces of casingswithin which the sensor electrodes reside, by face sheets applied overthe sensor electrodes or any casings, etc. In some embodiments, thesensing region 170 has a rectangular shape when projected onto an inputsurface of the input device 100.

The input device 100 may utilize any combination of sensor componentsand sensing technologies to detect user input in the sensing region 170.The input device 100 comprises a plurality of sensor electrodes 120 fordetecting user input. The input device 100 may include one or moresensor electrodes 120 that are combined to form sensor electrodes. Asseveral non-limiting examples, the input device 100 may use capacitive,elastive, resistive, inductive, magnetic acoustic, ultrasonic, and/oroptical techniques.

Some implementations are configured to provide images that span one,two, three, or higher dimensional spaces. Some implementations areconfigured to provide projections of input along particular axes orplanes.

In some resistive implementations of the input device 100, a flexibleand conductive first layer is separated by one or more spacer elementsfrom a conductive second layer. During operation, one or more voltagegradients are created across the layers. Pressing the flexible firstlayer may deflect it sufficiently to create electrical contact betweenthe layers, resulting in voltage outputs reflective of the point(s) ofcontact between the layers. These voltage outputs may be used todetermine positional information.

In some inductive implementations of the input device 100, one or moresensor electrodes 120 pickup loop currents induced by a resonating coilor pair of coils. Some combination of the magnitude, phase, andfrequency of the currents may then be used to determine positionalinformation.

In some capacitive implementations of the input device 100, voltage orcurrent is applied to create an electric field. Nearby input objectscause changes in the electric field, and produce detectable changes incapacitive coupling that may be detected as changes in voltage, current,or the like.

Some capacitive implementations utilize arrays or other regular orirregular patterns of capacitive sensor electrodes 120 to createelectric fields. In some capacitive implementations, separate sensorelectrodes 120 may be ohmically shorted together to form larger sensorelectrodes. Some capacitive implementations utilize resistive sheets,which may be uniformly resistive.

As discussed above, some capacitive implementations utilize“self-capacitance” (or “absolute capacitance”) sensing methods based onchanges in the capacitive coupling between sensor electrodes 120 and aninput object. In one embodiment, processing system 110 is configured todrive a voltage with known amplitude onto the sensor electrode 120 andmeasure the amount of charge required to charge the sensor electrode tothe driven voltage. In other embodiments, processing system 110 isconfigured to drive a known current and measure the resulting voltage.In various embodiments, an input object near the sensor electrodes 120alters the electric field near the sensor electrodes 120, thus changingthe measured capacitive coupling. In one implementation, an absolutecapacitance sensing method operates by modulating sensor electrodes 120with respect to a reference voltage (e.g. system ground) using amodulated signal, and by detecting the capacitive coupling between thesensor electrodes 120 and input objects 140.

Additionally as discussed above, some capacitive implementations utilize“mutual capacitance” (or “transcapacitance”) sensing methods based onchanges in the capacitive coupling between sensing electrodes. Invarious embodiments, an input object 140 near the sensing electrodesalters the electric field between the sensing electrodes, thus changingthe measured capacitive coupling. In one implementation, atranscapacitive sensing method operates by detecting the capacitivecoupling between one or more transmitter sensing electrodes (also“transmitter electrodes”) and one or more receiver sensing electrodes(also “receiver electrodes”) as further described below. Transmittersensing electrodes may be modulated relative to a reference voltage(e.g., system ground) to transmit a transmitter signals. Receiversensing electrodes may be held substantially constant relative to thereference voltage to facilitate receipt of resulting signals. Aresulting signal may comprise effect(s) corresponding to one or moretransmitter signals, and/or to one or more sources of environmentalinterference (e.g. other electromagnetic signals). Sensing electrodesmay be dedicated transmitter electrodes or receiver electrodes, or maybe configured to both transmit and receive.

In FIG. 1, the processing system 110 is shown as part of the inputdevice 100. The processing system 110 is configured to operate thehardware of the input device 100 to detect input in the sensing region170. The processing system 110 comprises parts of or all of one or moreintegrated circuits (ICs) and/or other circuitry components. Forexample, a processing system for a mutual capacitance sensor device maycomprise transmitter circuitry configured to transmit signals withtransmitter sensor electrodes, and/or receiver circuitry configured toreceive signals with receiver sensor electrodes. In some embodiments,the processing system 110 also comprises electronically-readableinstructions, such as firmware code, software code, and/or the like. Insome embodiments, components composing the processing system 110 arelocated together, such as near sensor electrode(s) 120 of the inputdevice 100. In other embodiments, components of processing system 110are physically separate with one or more components close to sensorelectrode(s) 120 of input device 100, and one or more componentselsewhere. For example, the input device 100 may be a peripheral coupledto a desktop computer, and the processing system 110 may comprisesoftware configured to run on a central processing unit of the desktopcomputer and one or more ICs (perhaps with associated firmware) separatefrom the central processing unit. As another example, the input device100 may be physically integrated in a phone, and the processing system110 may comprise circuits and firmware that are part of a main processorof the phone. In some embodiments, the processing system 110 isdedicated to implementing the input device 100. In other embodiments,the processing system 110 also performs other functions, such asoperating display screens, driving haptic actuators, etc.

The processing system 110 may be implemented as a set of modules thathandle different functions of the processing system 110. Each module maycomprise circuitry that is a part of the processing system 110,firmware, software, or a combination thereof. In various embodiments,different combinations of modules may be used. Example modules includehardware operation modules for operating hardware such as sensorelectrodes and display screens, data processing modules for processingdata such as sensor signals and positional information, and reportingmodules for reporting information. Further example modules includesensor operation modules configured to operate sensor electrodes 120 todetect input, identification modules configured to identify gesturessuch as mode changing gestures, and mode changing modules for changingoperation modes. Processing system 110 may also comprise one or morecontrollers.

In some embodiments, the processing system 110 responds to user input(or lack of user input) in the sensing region 170 directly by causingone or more actions. Example actions include changing operation modes,as well as GUI actions such as cursor movement, selection, menunavigation, and other functions. In some embodiments, the processingsystem 110 provides information about the input (or lack of input) tosome part of the electronic system (e.g. to a central processing systemof the electronic system that is separate from the processing system110, if such a separate central processing system exists). In someembodiments, some part of the electronic system processes informationreceived from the processing system 110 to act on user input, such as tofacilitate a full range of actions, including mode changing actions andGUI actions.

For example, in some embodiments, the processing system 110 operates thesensor electrode(s) 120 of the input device 100 to produce electricalsignals indicative of input (or lack of input) in the sensing region170. The processing system 110 may perform any appropriate amount ofprocessing on the electrical signals in producing the informationprovided to the electronic system. For example, the processing system110 may digitize analog electrical signals obtained from the sensorelectrodes 120. As another example, the processing system 110 mayperform filtering or other signal conditioning. As yet another example,the processing system 110 may subtract or otherwise account for abaseline, such that the information reflects a difference between theelectrical signals and the baseline. As yet further examples, theprocessing system 110 may determine positional information, recognizeinputs as commands, recognize handwriting, and the like.

“Positional information” as used herein broadly encompasses absoluteposition, relative position, velocity, acceleration, and other types ofspatial information. Exemplary “zero-dimensional” positional informationincludes near/far or contact/no contact information. Exemplary“one-dimensional” positional information includes positions along anaxis. Exemplary “two-dimensional” positional information includesmotions in a plane. Exemplary “three-dimensional” positional informationincludes instantaneous or average velocities in space. Further examplesinclude other representations of spatial information. Historical dataregarding one or more types of positional information may also bedetermined and/or stored, including, for example, historical data thattracks position, motion, or instantaneous velocity over time.

In some embodiments, the input device 100 is implemented with additionalinput components that are operated by the processing system 110 or bysome other processing system. These additional input components mayprovide redundant functionality for input in the sensing region 170, orsome other functionality. FIG. 1 shows buttons 130 near the sensingregion 170 that can be used to facilitate selection of items using theinput device 100. Other types of additional input components includesliders, balls, wheels, switches, and the like. Conversely, in someembodiments, the input device 100 may be implemented with no other inputcomponents.

In some embodiments, the input device 100 comprises a touch screeninterface, and the sensing region 170 overlaps at least part of anactive area of a display screen of the display device 160. For example,the input device 100 may comprise substantially transparent sensorelectrodes 120 overlaying the display screen and provide a touch screeninterface for the associated electronic system. The display screen maybe any type of dynamic display capable of displaying a visual interfaceto a user, and may include any type of light emitting diode (LED),organic LED (OLED), cathode ray tube (CRT), liquid crystal display(LCD), plasma, electroluminescence (EL), or other display technology.The input device 100 and the display device 160 may share physicalelements. For example, some embodiments may utilize some of the sameelectrical components for displaying and sensing. As another example,the display device 160 may be operated in part or in total by theprocessing system 110.

It should be understood that while many embodiments of the presenttechnology are described in the context of a fully functioningapparatus, the mechanisms of the present technology are capable of beingdistributed as a program product (e.g., software) in a variety of forms.For example, the mechanisms of the present technology may be implementedand distributed as a software program on information bearing media thatare readable by electronic processors (e.g., non-transitorycomputer-readable and/or recordable/writable information bearing mediareadable by the processing system 110). Additionally, the embodiments ofthe present technology apply equally regardless of the particular typeof medium used to carry out the distribution. Examples ofnon-transitory, electronically readable media include various discs,memory sticks, memory cards, memory modules, and the like.Electronically readable media may be based on flash, optical, magnetic,holographic, or any other storage technology.

Exemplary Sensor Electrode Implementations

FIGS. 2 and 3 illustrate portions of exemplary sensor electrodeimplementations, according to embodiments described herein.Specifically, implementation 200 (FIG. 2) illustrates a portion of apattern of sensor electrodes configured to sense in a sensing region 170associated with the pattern, according to several embodiments. Forclarity of illustration and description, FIG. 2 shows the sensorelectrodes in a pattern of simple rectangles, and does not show variousassociated components. This pattern of sensing electrodes comprises afirst plurality of sensor electrodes 205 (e.g., 205-1, 205-2, 205-3,205-4), and a second plurality of sensor electrodes 215 (e.g., 215-1,215-2, 215-3, 215-4). The sensor electrodes 205, 215 are each examplesof the sensor electrodes 120 discussed above. In one embodiment,processing system 110 operates the first plurality of sensor electrodes205 as a plurality of transmitter electrodes, and the second pluralityof sensor electrodes 215 as a plurality of receiver electrodes. Inanother embodiment, processing system 110 operates the first pluralityof sensor electrodes 205 and the second plurality of sensor electrodes215 as absolute capacitive sensing electrodes.

The first plurality of sensor electrodes 205 and the second plurality ofsensor electrodes 215 are typically ohmically isolated from each other.That is, one or more insulators separate the first plurality of sensorelectrodes 205 and the second plurality of sensor electrodes 215 andprevent them from electrically shorting to each other. In someembodiments, the first plurality of sensor electrodes 205 and the secondplurality of sensor electrodes 215 may be disposed on a common layer.The pluralities of sensor electrodes 205, 215 may be electricallyseparated by insulative material disposed between them at cross-overareas; in such constructions, the first plurality of sensor electrodes205 and/or the second plurality of sensor electrodes 215 may be formedwith jumpers connecting different portions of the same electrode. Insome embodiments, the first plurality of sensor electrodes 205 and thesecond plurality of sensor electrodes 215 are separated by one or morelayers of insulative material. In some embodiments, the first pluralityof sensor electrodes 205 and the second plurality of sensor electrodes215 are separated by one or more substrates; for example, they may bedisposed on opposite sides of the same substrate, or on differentsubstrates that are laminated together.

The pluralities of sensor electrodes 205, 215 may be formed into anydesired shapes. Moreover, the size and/or shape of the sensor electrodes205 may be different than the size and/or shape of the sensor electrodes215. Additionally, sensor electrodes 205, 215 located on a same side ofa substrate may have different shapes and/or sizes. In one embodiment,the first plurality of sensor electrodes 205 may be larger (e.g., havinga larger surface area) than the second plurality of sensor electrodes215, although this is not a requirement. In other embodiments, the firstand second pluralities of sensor electrodes 205, 215 may have a similarsize and/or shape.

In one embodiment, the first plurality of sensor electrodes 205 extendssubstantially in a first direction while the second plurality of sensorelectrodes 215 extends substantially in a second direction. For example,and as shown in FIG. 2, the first plurality of sensor electrodes 205extend in one direction, while the second plurality of sensor electrodes215 extend in a direction substantially orthogonal to the sensorelectrodes 205. Other orientations are also possible (e.g., parallel orother relative orientations).

In some embodiments, both the first and second pluralities of sensorelectrodes 205, 215 are located outside of a plurality (or displaystack) of layers that together form the display device 160. One exampleof a display stack may include layers such as a lens layer, a one ormore polarizer layers, a color filter layer, one or more displayelectrodes layers, a display material layer, a thin-film transistor(TFT) glass layer, and a backlight layer. However, other implementationsof a display stack are possible. In other embodiments, one or both ofthe first and second pluralities of sensor electrodes 205, 215 arelocated within the display stack, whether included as part of adisplay-related layer or a separate layer. For example, Vcom electrodeswithin a particular display electrode layer can be configured to performboth display updating and capacitive sensing.

Implementation 300 of FIG. 3 illustrates a portion of a pattern ofsensor electrodes configured to sense in sensing region 170, accordingto several embodiments. For clarity of illustration and description,FIG. 3 shows the sensor electrodes 120 in a pattern of simple rectanglesand does not show other associated components. The exemplary patterncomprises an array of sensor electrodes 120 _(X,Y) arranged in X columnsand Y rows, wherein X and Y are positive integers, although one of X andY may be zero. It is contemplated that the pattern of sensor electrodes120 may have other configurations, such as polar arrays, repeatingpatterns, non-repeating patterns, a single row or column, or othersuitable implementation. Further, in various embodiments the number ofsensor electrodes 120 may vary from row to row and/or column to column.In one embodiment, at least one row and/or column of sensor electrodes120 is offset from the others, such it extends further in at least onedirection than the others. The sensor electrodes 120 is coupled to theprocessing system 110 and utilized to determine the presence (or lackthereof) of an input object in the sensing region 170.

In a first mode of operation, the implementation of sensor electrodes120 (120 _(1,1), 120 _(2,1), 120 _(3,1), . . . , 120 _(X,Y)) may beutilized to detect the presence of an input object via absolute sensingtechniques. That is, processing system 110 is configured to modulatesensor electrodes 120 to acquire measurements of changes in capacitivecoupling between the modulated sensor electrodes 120 and an input objectto determine the position of the input object. Processing system 110 isfurther configured to determine changes of absolute capacitance based ona measurement of resulting signals received with sensor electrodes 120which are modulated.

In some embodiments, the implementation 300 includes one or more gridelectrodes (not shown) that are disposed between at least two of thesensor electrodes 120. The grid electrode(s) may at least partiallycircumscribe the plurality of sensor electrodes 120 as a group, and mayalso, or in the alternative, completely or partially circumscribe one ormore of the sensor electrodes 120. In one embodiment, the grid electrodeis a planar body having a plurality of apertures, where each aperturecircumscribes a respective one of the sensor electrodes 120. In otherembodiments, the grid electrode(s) comprise a plurality of segments thatmay be driven individually or in groups or two or more segments. Thegrid electrode(s) may be fabricated similar to the sensor electrodes120. The grid electrode(s), along with sensor electrodes 120, may becoupled to the processing system 110 utilizing conductive routing tracesand used for input object detection.

The sensor electrodes 120 are typically ohmically isolated from eachother, and are also ohmically isolated from the grid electrode(s). Thatis, one or more insulators separate the sensor electrodes 120 and gridelectrode(s) and prevent them from electrically shorting to each other.In some embodiments, the sensor electrodes 120 and grid electrode(s) areseparated by an insulative gap, which may be filled with an electricallyinsulating material, or may be an air gap. In some embodiments, thesensor electrodes 120 and the grid electrode(s) are vertically separatedby one or more layers of insulative material. In some other embodiments,the sensor electrodes 120 and the grid electrode(s) are separated by oneor more substrates; for example, they may be disposed on opposite sidesof the same substrate, or on different substrates. In yet otherembodiments, the grid electrode(s) may be composed of multiple layers onthe same substrate, or on different substrates. In one embodiment, afirst grid electrode may be formed on a first substrate (or a first sideof a substrate) and a second grid electrode may be formed on a secondsubstrate (or a second side of a substrate). For example, a first gridelectrode comprises one or more common electrodes disposed on athin-film transistor (TFT) layer of the display device 160 (FIG. 1) anda second grid electrode is disposed on the color filter glass of thedisplay device 160. The dimensions of the first and second gridelectrodes can be equal or differ in at least one dimension.

In a second mode of operation, the sensor electrodes 120 (120 _(1,1),120 _(2,1), 120 _(3,1), . . . , 120 _(X,Y)) may be utilized to detectthe presence of an input object via transcapacitive sensing techniqueswhen a transmitter signal is driven onto the grid electrode(s). That is,processing system 110 is configured to drive the grid electrode(s) witha transmitter signal and to receive resulting signals with each sensorelectrode 120, where a resulting signal comprising effects correspondingto the transmitter signal, which is utilized by the processing system110 or other processor to determine the position of the input object.

In a third mode of operation, the sensor electrodes 120 may be splitinto groups of transmitter and receiver electrodes utilized to detectthe presence of an input object via transcapacitive sensing techniques.That is, processing system 110 may drive a first group of sensorelectrodes 120 with a transmitter signal and receive resulting signalswith the second group of sensor electrodes 120, where a resulting signalcomprising effects corresponding to the transmitter signal. Theresulting signal is utilized by the processing system 110 or otherprocessor to determine the position of the input object.

The input device 100 may be configured to operate in any one of themodes described above. The input device 100 may also be configured toswitch between any two or more of the modes described above.

The areas of localized capacitive sensing of capacitive couplings may betermed “capacitive pixels,” “touch pixels,” “tixels,” etc. Capacitivepixels may be formed between an individual sensor electrode 120 and areference voltage in the first mode of operation, between the sensorelectrodes 120 and grid electrode(s) in the second mode of operation,and between groups of sensor electrodes 120 used as transmitter andreceiver electrodes (e.g., implementation 200 of FIG. 2). The capacitivecoupling changes with the proximity and motion of input objects in thesensing region 170 associated with the sensor electrodes 120, and thusmay be used as an indicator of the presence of the input object in thesensing region of the input device 100.

In some embodiments, the sensor electrodes 120 are “scanned” todetermine these capacitive couplings. That is, in one embodiment, one ormore of the sensor electrodes 120 are driven to transmit transmittersignals. Transmitters may be operated such that one transmitterelectrode transmits at one time, or such that multiple transmitterelectrodes transmit at the same time. Where multiple transmitterelectrodes transmit simultaneously, the multiple transmitter electrodesmay transmit the same transmitter signal and thereby produce aneffectively larger transmitter electrode. Alternatively, the multipletransmitter electrodes may transmit different transmitter signals. Forexample, multiple transmitter electrodes may transmit differenttransmitter signals according to one or more coding schemes that enabletheir combined effects on the resulting signals of receiver electrodesto be independently determined. In one embodiment, multiple transmitterelectrodes may simultaneously transmit the same transmitter signal whilethe receiver electrodes receive the effects and are measured accordingto a scanning scheme.

The sensor electrodes 120 configured as receiver sensor electrodes maybe operated singly or multiply to acquire resulting signals. Theresulting signals may be used to determine measurements of thecapacitive couplings at the capacitive pixels. Processing system 110 maybe configured to receive with the sensor electrodes 120 in a scanningfashion and/or a multiplexed fashion to reduce the number ofsimultaneous measurements to be made, as well as the size of thesupporting electrical structures. In one embodiment, one or more sensorelectrodes are coupled to a receiver of processing system 110 via aswitching element such as a multiplexer or the like. In such anembodiment, the switching element may be internal to processing system110 or external to processing system 110. In one or more embodiments,the switching elements may be further configured to couple a sensorelectrode 120 with a transmitter or other signal and/or voltagepotential. In one embodiment, the switching element may be configured tocouple more than one receiver electrode to a common receiver at the sametime.

In other embodiments, “scanning” sensor electrodes 120 to determinethese capacitive couplings comprises modulating one or more of thesensor electrodes and measuring an absolute capacitance of the one orsensor electrodes. In another embodiment, the sensor electrodes may beoperated such that more than one sensor electrode is driven and receivedwith at a time. In such embodiments, an absolute capacitive measurementmay be obtained from each of the one or more sensor electrodes 120simultaneously. In one embodiment, each of the sensor electrodes 120 aresimultaneously driven and received with, obtaining an absolutecapacitive measurement simultaneously from each of the sensor electrodes120. In various embodiments, processing system 110 may be configured toselectively modulate a portion of sensor electrodes 120. For example,the sensor electrodes may be selected based on, but not limited to, anapplication running on the host processor, a status of the input device,and an operating mode of the sensing device. In various embodiments,processing system 110 may be configured to selectively shield at least aportion of sensor electrodes 120 and to selectively shield or transmitwith the grid electrode(s) 122 while selectively receiving and/ortransmitting with other sensor electrodes 120.

A set of measurements from the capacitive pixels form a “capacitiveimage” (also “capacitive frame”) representative of the capacitivecouplings at the pixels. Multiple capacitive images may be acquired overmultiple time periods, and differences between them used to deriveinformation about input in the sensing region. For example, successivecapacitive images acquired over successive periods of time can be usedto track the motion(s) of one or more input objects entering, exiting,and within the sensing region.

In any of the above embodiments, multiple sensor electrodes 120 may beganged together such that the sensor electrodes 120 are simultaneouslymodulated or simultaneously received with. As compared to the methodsdescribed above, ganging together multiple sensor electrodes may producea coarse capacitive image that may not be usable to discern precisepositional information. However, a coarse capacitive image may be usedto sense presence of an input object. In one embodiment, the coarsecapacitive image may be used to move processing system 110 or the inputdevice 100 out of a “doze” mode or low-power mode. In one embodiment,the coarse capacitive image may be used to move a capacitive sensing ICout of a “doze” mode or low-power mode. In another embodiment, thecoarse capacitive image may be used to move at least one of a host ICand a display driver out of a “doze” mode or low-power mode. The coarsecapacitive image may correspond to the entire sensor area or only to aportion of the sensor area.

The background capacitance of the input device 100 is the capacitiveimage associated with no input object in the sensing region 170. Thebackground capacitance changes with the environment and operatingconditions, and may be estimated in various ways. For example, someembodiments take “baseline images” when no input object is determined tobe in the sensing region 170, and use those baseline images as estimatesof their background capacitances. The background capacitance or thebaseline capacitance may be present due to stray capacitive couplingbetween two sensor electrodes, where one sensor electrode is driven witha modulated signal and the other is held stationary relative to systemground, or due to stray capacitive coupling between a receiver electrodeand nearby modulated electrodes. In many embodiments, the background orbaseline capacitance may be relatively stationary over the time periodof a user input gesture.

Capacitive images can be adjusted for the background capacitance of theinput device 100 for more efficient processing. Some embodimentsaccomplish this by “baselining” measurements of the capacitive couplingsat the capacitive pixels to produce a “baselined capacitive image.” Thatis, some embodiments compare the measurements forming a capacitanceimage with appropriate “baseline values” of a “baseline image”associated with those pixels, and determine changes from that baselineimage.

In some touch screen embodiments, one or more of the sensor electrodes120 comprise one or more display electrodes used in updating the displayof the display screen. The display electrodes may comprise one or moreelements of the active matrix display such as one or more segments of asegmented Vcom electrode (common electrode(s)), a source drive line,gate line, an anode sub-pixel electrode or cathode pixel electrode, orany other suitable display element. These display electrodes may bedisposed on an appropriate display screen substrate. For example, thecommon electrodes may be disposed on the a transparent substrate (aglass substrate, TFT glass, or any other transparent material) in somedisplay screens (e.g., In-Plane Switching (IPS), Fringe Field Switching(FFS) or Plane to Line Switching (PLS) Organic Light Emitting Diode(OLED)), on the bottom of the color filter glass of some display screens(e.g., Patterned Vertical Alignment (PVA) or Multi-domain VerticalAlignment (MVA)), over an emissive layer (OLED), etc. In suchembodiments, the display electrode can also be referred to as a“combination electrode,” since it performs multiple functions. Invarious embodiments, each of the sensor electrodes 120 comprises one ormore common electrodes. In other embodiments, at least two sensorelectrodes 120 may share at least one common electrode. While thefollowing description may describe that sensor electrodes 120 and/orgrid electrode(s) comprise one or more common electrodes, various otherdisplay electrodes as describe above may also be used in conjunctionwith the common electrode or as an alternative to the common electrodes.In various embodiments, the sensor electrodes 120 and grid electrode(s)comprise the entire common electrode layer (Vcom electrode).

In various touch screen embodiments, the “capacitive frame rate” (therate at which successive capacitive images are acquired) may be the sameor be different from that of the “display frame rate” (the rate at whichthe display image is updated, including refreshing the screen toredisplay the same image). In various embodiments, the capacitive framerate is an integer multiple of the display frame rate. In otherembodiments, the capacitive frame rate is a fractional multiple of thedisplay frame rate. In yet further embodiments, the capacitive framerate may be any fraction or integer multiple of the display frame rate.In one or more embodiments, the display frame rate may change (e.g., toreduce power or to provide additional image data such as a 3D displayinformation) while touch frame rate maintains constant. In otherembodiment, the display frame rate may remain constant while the touchframe rate is increased or decreased.

Continuing to refer to FIG. 3, the processing system 110 coupled to thesensor electrodes 120 includes a sensing module 310 and optionally, adisplay driver module 320. The sensing module 310 includes circuitryconfigured to drive at least one of the sensor electrodes 120 forcapacitive sensing during periods in which input sensing is desired. Inone embodiment, the sensing module 310 is configured to drive amodulated signal onto the at least one sensor electrode 120 to detectchanges in absolute capacitance between the at least one sensorelectrode and an input object. In another embodiment, the sensing module310 is configured to drive a transmitter signal onto the at least onesensor electrode 120 to detect changes in a transcapacitance between theat least one sensor electrode and another sensor electrode 120. Themodulated and transmitter signals are generally varying voltage signalscomprising a plurality of voltage transitions over a period of timeallocated for input sensing. In various embodiments, the sensorelectrodes 120 and/or grid electrode(s) may be driven differently indifferent modes of operation. In one embodiment, the sensor electrodes120 and/or grid electrode(s) may be driven with signals (modulatedsignals, transmitter signals and/or shield signals) that may differ inany one of phase, amplitude, and/or shape. In various embodiments, themodulated signal and transmitter signal are similar in at least oneshape, frequency, amplitude, and/or phase. In other embodiments, themodulated signal and the transmitter signals are different in frequency,shape, phase, amplitude, and phase. The sensing module 310 may beselectively coupled one or more of the sensor electrodes 120 and/or thegrid electrode(s). For example, the sensing module 310 may be coupledselected portions of the sensor electrodes 120 and operate in either anabsolute or transcapacitive sensing mode. In another example, thesensing module 310 may be a different portion of the sensor electrodes120 and operate in either an absolute or transcapacitive sensing mode.In yet another example, the sensing module 310 may be coupled to all thesensor electrodes 120 and operate in either an absolute ortranscapacitive sensing mode.

The sensing module 310 is configured to operate the grid electrode(s) asa shield electrode that may shield sensor electrodes 120 from theelectrical effects of nearby conductors. In one embodiment, theprocessing system is configured to operate the grid electrode(s) as ashield electrode that may “shield” sensor electrodes 120 from theelectrical effects of nearby conductors, and to guard the sensorelectrodes 120 from grid electrode(s), at least partially reducing theparasitic capacitance between the grid electrode(s) and the sensorelectrodes 120. In one embodiment, a shielding signal is driven onto thegrid electrode(s). The shielding signal may be a ground signal, such asthe system ground or other ground, or any other constant voltage (i.e.,non-modulated) signal. In another embodiment, operating the gridelectrode(s) as a shield electrode may comprise electrically floatingthe grid electrode. In one embodiment, grid electrode(s) are able tooperate as an effective shield electrode while being electrode floateddue to its large coupling to the other sensor electrodes. In otherembodiment, the shielding signal may be referred to as a “guardingsignal” where the guarding signal is a varying voltage signal having atleast one of a similar phase, frequency, and amplitude as the modulatedsignal driven on to the sensor electrodes. In one or more embodiment,routing traces may be shielded from responding to an input object due torouting beneath the grid electrode(s) and/or sensor electrodes 120, andtherefore may not be part of the active sensor electrodes, shown assensor electrodes 120.

In one or more embodiments, capacitive sensing (or input sensing) anddisplay updating may occur during at least partially overlappingperiods. For example, as a common electrode is driven for displayupdating, the common electrode may also be driven for capacitivesensing. In another embodiment, capacitive sensing and display updatingmay occur during non-overlapping periods, also referred to asnon-display update periods. In various embodiments, the non-displayupdate periods may occur between display line update periods for twodisplay lines of a display frame and may be at least as long in time asthe display update period. In such embodiments, the non-display updateperiod may be referred to as a “long horizontal blanking period,” “longh-blanking period” or a “distributed blanking period,” where theblanking period occurs between two display updating periods and is atleast as long as a display update period. In one embodiment, thenon-display update period occurs between display line update periods ofa frame and is long enough to allow for multiple transitions of thetransmitter signal to be driven onto the sensor electrodes 120. In otherembodiments, the non-display update period may comprise horizontalblanking periods and vertical blanking periods. Processing system 110may be configured to drive sensor electrodes 120 for capacitive sensingduring any one or more of or any combination of the differentnon-display update times. Synchronization signals may be shared betweensensing module 310 and display driver module 320 to provide accuratecontrol of overlapping display updating and capacitive sensing periodswith repeatably coherent frequencies and phases. In one embodiment,these synchronization signals may be configured to allow the relativelystable voltages at the beginning and end of the input sensing period tocoincide with display update periods with relatively stable voltages(e.g., near the end of a input integrator reset time and near the end ofa display charge share time). A modulation frequency of a modulated ortransmitter signal may be at a harmonic of the display line update rate,where the phase is determined to provide a nearly constant chargecoupling from the display elements to the receiver electrode, allowingthis coupling to be part of the baseline image.

The sensing module 310 includes circuitry configured to receiveresulting signals with the sensor electrodes 120 and/or gridelectrode(s) comprising effects corresponding to the modulated signalsor the transmitter signals during periods in which input sensing isdesired. The sensing module 310 may determine a position of the inputobject in the sensing region 170 or may provide a signal includinginformation indicative of the resulting signal to another module orprocessor, for example, a determination module 330 or a processor of anassociated electronic device 150 (i.e., a host processor), fordetermining the position of the input object in the sensing region 170.

The display driver module 320 may be included in or separate from theprocessing system 110. The display driver module 320 includes circuitryconfigured to provide display image update information to the display ofthe display device 160 during non-sensing (e.g., display updating)periods.

In one embodiment, the processing system 110 comprises a firstintegrated controller comprising the display driver module 320 and atleast a portion of the sensing module 310 (i.e., transmitter moduleand/or receiver module). In another embodiment, the processing system110 comprises a first integrated controller comprising the displaydriver module 320 and a second integrated controller comprising thesensing module 310. In yet another embodiment, the processing systemcomprises a first integrated controller comprising display driver module320 and a first portion of the sensing module 310 (e.g., one of atransmitter module and a receiver module) and a second integratedcontroller comprising a second portion of the sensing module 310 (e.g.,the other one of the transmitter and receiver modules). In thoseembodiments comprising multiple integrated circuits, a synchronizationmechanism may be coupled between them, configured to synchronize displayupdating periods, sensing periods, transmitter signals, display updatesignals, and the like.

Exemplary Sensing Implementations

FIG. 4 illustrates an exemplary processing system for applying selectedmixing periods for different sensing frequencies, according toembodiments described herein. More specifically, implementation 400provides one possible implementation of the processing system 110discussed above. Further, the implementation 400 is capable of use inconjunction with various embodiments discussed herein, such as theimplementations 200, 300 of sensor electrodes discussed above withrespect to FIGS. 2 and 3.

Within implementation 400, the sensing module 310 comprises a pluralityof sensor electrode groups 405. Each group G1, G2, G3, G4, . . . , GN ofthe plurality of sensor electrode groups 405 corresponds to at least onesensor electrode of the plurality of sensor electrodes that are coupledwith the processing system 110. The processing system 110 operates toperform capacitive sensing by operating selected groups of the groupsG1, G2, G3, G4, . . . , GN of sensor electrodes to transmit capacitivesensing signals and/or to receive resulting signals. For example, in atranscapacitive sensing implementation, a first group G1 of one or moresensor electrodes is driven with a capacitive sensing signal, and asecond group G2 of one or more other sensor electrodes receivesresulting signals. In another example, in an absolute capacitive sensingimplementation, a particular group G1 of sensor electrodes is drivenwith a capacitive sensing signal and is also used to receive theresulting signals.

The sensing module 310 further comprises a plurality of predefinedsensing frequencies 415. Capacitive sensing signals that are generatedby the processing system 110 and subsequently driven onto selectedsensor electrodes are generally time-varying signals having a frequencycorresponding to a selected one of the sensing frequencies f_(SENS,1),f_(SENS,2), f_(SENS,3), . . . , f_(SENS,K). Further, during operation ofthe processing system 110, the frequency of the generated capacitivesensing signals may be transitioned between selected ones of the sensingfrequencies f_(SENS,1), f_(SENS,2), f_(SENS,3), . . . , f_(SENS,K) toavoid interference and thus to improve sensing performance. In somecases, the transition between sensing frequencies f_(SENS,1),f_(SENS,2), f_(SENS,3), . . . , f_(SENS,K) is performed responsive tointerference measurements performed by the processing system 110.

In some embodiments, the predefined sensing frequencies f_(SENS,1),f_(SENS,2), f_(SENS,3), . . . , f_(SENS,K) are selected based on one ormore slowest sensor electrodes 410 that are identified within theplurality of sensor electrodes. The one or more slowest sensorelectrodes 410 are associated with the relatively longest RC timeconstants, which causes the one or more slowest sensor electrodes 410 tosettle more slowly when driven with various signals (such as thegenerated capacitive sensing signals). For example, within an inputdevice, the slowest sensor electrodes 410 may generally correspond tothose sensor electrodes that are spatially furthest from the processingsystem 110 and which are connected with the longest and/or mostcircuitous conductive traces. In such embodiments, the predefinedsensing frequencies f_(SENS,1), f_(SENS,2), f_(SENS,3), . . . ,f_(SENS,K) are selected such that generating capacitive sensing signalswith the highest sensing frequency produces acceptable sensingperformance even for the slowest sensor electrodes 410.

The sensing module 310 further comprises a plurality of predefinedmixing periods 420 (or “mixing window pulse widths”) that are used fordemodulating resulting signals received by the sensor electrodes (i.e.,according to the selected sensor electrode groups G1, G2, . . . , GN).Each mixing period T_(MIX,1), T_(MIX,2), T_(MIX,3), . . . , T_(MIX,K) ofthe plurality of predefined mixing periods 420 corresponds to aparticular sensing frequency f_(SENS,1), f_(SENS,2), f_(SENS,3), . . . ,f_(SENS,K).

In some embodiments, the mixing periods T_(MIX,1), T_(MIX,2), T_(MIX,3),. . . , T_(MIX,K) are selected such that a plurality of average currentvalues 430 (i.e., average current values I_(AVG,1), I_(AVG,2),I_(AVG,3), . . . , I_(AVG,K)) corresponding to a plurality of acquiredbaseline capacitive measurements 425 (i.e., baseline capacitivemeasurements B(1), B(2), . . . , B(K) performed with no input objectpresent) have a predefined relationship with the different sensingfrequencies f_(SENS,1), f_(SENS,2), f_(SENS,3), . . . , f_(SENS,K).Thus, for any transitions between different ones of the predefinedsensing frequencies f_(SENS,1), f_(SENS,2), f_(SENS,3), . . . ,f_(SENS,K) during operation of the processing system 110, thecorresponding average current values I_(AVG,1), I_(AVG,2), I_(AVG,3), .. . , I_(AVG,K) will have a predictable relationship, which simplifiesany correction or compensation that may be needed (e.g., scaling orshifting) to capacitive sensing measurements. In this way, theprocessing system 110 avoids the need for calibrating and storingcompensation values for each individual sensor electrode or capacitivepixel, which reduces memory requirements and associated cost and size.

In one embodiment, the various mixing periods T_(MIX,1), T_(MIX,2),T_(MIX,3), . . . , T_(MIX,K) are selected such that the correspondingaverage current values I_(AVG,1), I_(AVG,2), I_(AVG,3), . . . ,I_(AVG,K) are substantially identical. In such a case, the processingsystem 110 need not perform any correction or compensation to thecapacitive sensing measurements when transitioning between differentsensing frequencies f_(SENS,1), f_(SENS,2), f_(SENS,3), . . . ,f_(SENS,K). In another embodiment, the plurality of average currentvalues 430 has a substantially linear relation to the changes to thesensing frequency f_(SENS,1), f_(SENS,2), f_(SENS,3), . . . ,f_(SENS,K), which requires only a relatively simple correction orcompensation by the processing system 110.

In some embodiments, the implementation 400 comprises one or moreswitching elements 435 coupled with one or more sensor electrodescorresponding to the groups G1, G2, G3, G4, . . . , GN of sensorelectrodes. The processing system 110 generally operates the switchingelements 435 such that the plurality of average current values 430(i.e., average current values I_(AVG,1), I_(AVG,2), I_(AVG,3), . . . ,I_(AVG,K)) have a predefined relationship with the different sensingfrequencies f_(SENS,1), f_(SENS,2), f_(SENS,3), . . . f_(SENS,K). Insome embodiments, the switching elements 435 are in a conducting (or“on”) state during mixing periods selected from the mixing periodsT_(MIX,1), T_(MIX,2), T_(MIX,3), . . . , T_(MIX,K), and in anon-conducting (or “off”) state during non-mixing periods. In this way,the switching elements 435 couple the associated sensor electrodeswithin the sensing implementation during the mixing periods, anddecouple the sensor electrodes during non-mixing periods. The operationand timing of the switching elements 435 is discussed in greater detailwith respect to FIGS. 9, 10, and 11 below. Some non-limiting examples ofthe one or more switching elements 435 include transistors andmultiplexers. Further, although depicted as external to the processingsystem 110, the switching elements 435 may alternately be includedwithin the processing system 110.

FIG. 5 is a schematic diagram of a sensing implementation 500 (or“implementation”) for applying selected mixing periods for differentsensing frequencies, according to embodiments described herein. Theimplementation 500 is capable of use in conjunction with variousembodiments discussed herein, such as the implementations 200, 300 ofsensor electrodes discussed above with respect to FIGS. 2 and 3 and theprocessing system depicted in FIG. 4.

The implementation 500 comprises a voltage source 505 that generates acapacitive sensing signal having a voltage waveform V_(TX). The voltagewaveform V_(TX) may have any suitable shape, and the frequency ofvoltage waveform V_(TX) is controlled based on which predefined sensingfrequency (e.g., f_(SENS,1), f_(SENS,2), f_(SENS,3), . . . f_(SENS,K) ofFIG. 4) is selected. The voltage source 505 drives the capacitivesensing signal onto a sensor electrode 510, which is represented as afirst-order model having a transmitter resistance R_(TX) and atransmitter capacitance C_(TX). Based on a transcapacitance C_(T)between sensor electrode 510 and sensor electrode 515, the sensorelectrode 515 receives resulting signals which are provided to receivercircuitry 520. The sensor electrode 515 is also represented as afirst-order model having a receiver resistance R_(RX) and a receivercapacitance C_(RX). However, the principles discussed herein also applyto more complex (e.g., distributed) modeling of the plurality of sensorelectrodes. Further, although described in terms of a transcapacitiveimplementation having two separate sensor electrodes 510, 515, theprinciples discussed herein also apply to absolute capacitiveimplementations in which the same sensor electrode(s) are used totransmit and receive capacitive sensing signals.

As shown, the receiver circuitry 520 comprises an amplifier 525 (or“op-amp”), a current conveyor (or “current mirror”) 530, and ademodulator (or “mixer”) 535. The input current I_(IN) represents theresulting signals received by the sensor electrode 515, and is mirroredby the current conveyor 530 (with any suitable gain value A) as anoutput current I_(OUT). Based on a received demodulation signal, thedemodulator 535 downconverts the output current I_(OUT) having higher(RF) frequencies to a demodulated current I_(MIX), which may be filteredby subsequent circuitry to have approximately direct current (DC) levels(i.e., having substantially no frequency component). As shown, a voltagesource 540 generates the demodulation signal with a voltage waveformV_(MIX) that is based on which predefined mixing period (e.g.,T_(MIX,1), T_(MIX,2), T_(MIX,3), . . . , T_(MIX,K) of FIG. 4) isselected.

The demodulator 535 is generally a continuous-time demodulator havingany suitable implementation, such as a square-wave mixer, harmonicrejection mixer, or sinusoidal mixer. Generally, use of acontinuous-time demodulator 535 allows a relatively simple hardwareimplementation of the receiver circuitry 520, while avoiding the needfor calibrating and storing compensation values for each individualsensor electrode or capacitive pixel. In some embodiments, thedemodulator 535 receives a three-level demodulation signal having apositive level, a negative level, and a zero level.

FIG. 6 illustrates a method 600 for applying selected mixing periods fordifferent sensing frequencies, according to embodiments describedherein. The method 600 is capable of use in conjunction with variousembodiments discussed herein, such as the processing system depicted inFIG. 4, or any other suitable processing system.

Method 600 begins at block 605, where the processing system drives, ontoa first group of a plurality of sensor electrodes, a first capacitivesensing signal having a predefined first sensing frequency. At block615, the processing system acquires, based on the driven firstcapacitive sensing signal, first capacitive measurements of resultingsignals received by a second group of the plurality of sensorelectrodes. Each of the first group and the second group comprises oneor more sensor electrodes. In some absolute capacitive sensingimplementations, the first group and the second group are the same. Insome transcapacitive sensing implementations, the first group and thesecond group are different. During block 615, the processing systemfurther applies a first demodulation signal having a predefined firstmixing period defined within a sensing period associated with the firstsensing frequency.

At block 625, the processing system drives, onto a third group of theplurality of sensor electrodes, a second capacitive sensing signalhaving a second sensing frequency different than the first sensingfrequency. The third group may be the same as the first group, but thisis not a requirement. In some cases, the transition between the firstsensing frequency and the second sensing frequency is performedresponsive to detected interference at the first sensing frequency. Atblock 635, the processing system acquires, based on the driven secondcapacitive sensing signal, second capacitive measurements of resultingsignals received by a fourth group of the plurality of sensorelectrodes. The fourth group may be the same as the second group, butthis is not a requirement. During block 635, the processing systemfurther applies a second demodulation signal having a predefined secondmixing period different than the first mixing period. In someembodiments, the first and second mixing periods are selected such thata first average current value for a first baseline capacitivemeasurement at the first sensing frequency has a linear relation with asecond average current value for a second baseline capacitivemeasurement at the second sensing frequency. Method 600 ends followingcompletion of block 635.

FIG. 7 is a diagram 700 illustrating exemplary operation of a sensingimplementation without performing adjustments of the mixing period,according to embodiments described herein. Generally, the diagram 700represents operation of the sensing implementation 500 depicted in FIG.5, or any other suitable processing system.

Diagram 700 includes a plot 705 of voltage waveform V_(TX) in volts (V)over time in microseconds (us). As shown, the voltage waveform V_(TX) isa square wave having a sensing frequency of 300 kilohertz (kHz) andalternating between 3V and 0V levels. Diagram 700 further includes agraph 710, which includes a plot 720 of demodulated current I_(MIX)(i.e., the output current from the demodulator), and a plot 725 of inputcurrent I_(IN) (i.e., the input current from received resultingsignals). Diagram 700 further includes a plot 715 of voltage waveformV_(MIX), which as shown is a square wave operating at the sensingfrequency of 300 kHz and alternating between two levels (i.e., 1V and−1V levels).

Each predefined sensing period forms a plurality of sensing cycles740-1, 740-2, and so forth. Each sensing cycle 740-1, 740-2 includes apositive sensing half-cycle 730 and a negative sensing half-cycle 735.Each sensing cycle 740-1, 740-2 has a duration of approximately 3.33 us,which corresponds to the 300 kHz sensing frequency. As shown, during thepositive sensing half-cycle 730, the demodulator applies the positivelevel of the demodulation signal (i.e., 1V), and during the negativesensing half-cycle 735, the demodulator applies the negative level ofthe demodulation signal (i.e., −1V) such that the multiplicative productof the input current I_(IN) and the voltage waveform V_(MIX) produces asignal having a non-zero average value over a single sensing period oran integer number of sensing periods. Within plot 715, the mixing periodT_(MIX) equals the length of each sensing half-cycle 730, 735 (here,approximately 1.67 us).

FIG. 8 is a diagram 800 illustrating exemplary operation of a sensingimplementation with adjustments performed to the mixing period,according to embodiments described herein.

Diagram 800 includes the plot 705 of voltage waveform V_(TX), which is asquare wave having a sensing frequency of 300 kilohertz (kHz) andalternating between 3V and 0V levels. Each sensing cycle 740-1, 740-2has a duration of approximately 3.33 us, which corresponds to the 300kHz sensing frequency. Diagram 800 further includes a graph 805, whichincludes a plot 815 of demodulated current I_(MIX), and a plot 820 ofinput current I_(IN). Diagram 800 further includes a plot 810 of voltagewaveform V_(MIX), which as shown is a three-level demodulation signalhaving a positive level (i.e., 1V level), a negative level (i.e., −1V),and a zero level.

Sensing cycle 740-1 comprises a positive sensing half-cycle 730 duringwhich the demodulator applies the positive level of the demodulationsignal, a negative sensing half-cycle 735 during which the demodulatorapplies the negative level of the demodulation signal, and predefinedperiods 825-1, 825-2 (or non-mixing periods) during which the zero levelis applied. Predefined period 825-1 occurs between the positive sensinghalf-cycle 730 and the negative sensing half-cycle 735, and predefinedperiod 825-2 occurs between the negative sensing half-cycle 735 and apositive sensing half-cycle of the subsequent sensing cycle 740-2. Asshown, the mixing period T_(MIX) represents the amount of time duringwhich the demodulation signal is at each of the positive level and thenegative level at a given sensing frequency. Within plot 810, the mixingperiod T_(MIX) is approximately 1.06 us and each predefined period825-1, 825-2 is approximately 0.60 us.

In some embodiments, the durations of the mixing period T_(MIX) (orsensing half-cycles 730, 735) and the predefined periods 825-1, 825-2are controlled such that, for baseline capacitive measurements acquiredat the different predefined sensing frequencies, the average currentvalues acquired during each sensing half-cycle 730, 735 have apredefined relationship. In one non-limiting example, the averagecurrent values may be the same for the different sensing frequencies. Inanother non-limiting example, the average current values may have alinear relationship based on the difference between the sensingfrequencies.

Referring also to the sensing implementation 500 depicted in FIG. 5, andassuming that R_(TX)=0 and C_(TX)=0, the average current value during asensing half-cycle can be represented as:

$\begin{matrix}{I_{MIX} = {C_{T} \cdot V_{TX} \cdot f_{SENS} \cdot {\quad{\left\lbrack {\left( {1 - e^{- \frac{T_{MIX}}{R_{RX} \cdot {({C_{T} + C_{RX}})}}}} \right) \cdot \left( {1 + {\tanh \frac{1}{{4 \cdot f_{SENS} \cdot R_{RX} \cdot \left( {C_{T} + C_{RX}} \right)}\;}}} \right)} \right\rbrack.}}}} & (1)\end{matrix}$

For the case where T_(MIX)=½f_(SENS) (such as depicted in FIG. 7), theaverage current value during a sensing half-cycle can be represented as:

$\begin{matrix}{{I_{MIX} = {{2 \cdot C_{T} \cdot V_{TX} \cdot f_{SENS} \cdot \tanh}\frac{1}{4 \cdot f_{SENS} \cdot R_{RX} \cdot \left( {C_{T} + C_{RX}} \right)}}},} & (2)\end{matrix}$

which simplifies to I_(MIX)=2·C_(T)·V_(TX)·f_(SENS) (a linear result) asthe term (R_(RX)·(C_(T)+C_(RX))) approaches zero. Thus, the outputcurrent I_(OUT) should scale linearly with changes to sensing frequencyf_(SENS). The measured capacitance may thus be determined as

$C_{T} = {\frac{I_{OUT}}{2 \cdot V_{TX} \cdot f_{SENS}}.}$

upon transitioning from a first sensing frequency f_(SENS,1) to a secondsensing frequency f_(SENS,2), the ratio of

$\frac{I_{{MIX},1}}{I_{{MIX},2}}$

equals the ratio of

$\frac{f_{{SENS},1}}{f_{{SENS},2}}.$

In other words, I_(MIX,1) may be linearly scaled by

$\frac{f_{{SENS},2}}{f_{{SENS},1}}$

to obtain I_(MIX,2).

As a baseline reference, if T_(MIX) is maintained constant with changesin sensing frequency f_(SENS), the average current value ideally remainsconstant for each T_(MIX) window. Thus, the performance improvement forthe case where a constant T_(MIX) is applied to Equation (1), over thecase where T_(MIX)=½f_(SENS) shown in Equation (2) can be representedas:

$\begin{matrix}{{\frac{\Delta \; I_{{MIX}\; {({{eq}.\mspace{11mu} 2})}}}{\Delta \; I_{{MIX}\mspace{11mu} {({{eq}.\mspace{11mu} 1})}}} = \frac{2}{1 - e^{- \frac{T_{MIX}}{R_{RX} \cdot {({C_{T} + C_{RX}})}}}}},{{{where}\mspace{14mu} \Delta \; I_{MIX}} = {{I_{{MIX},1} \cdot \left\lbrack \frac{f_{{SENS},2}}{f_{{SENS},1}} \right\rbrack} - I_{{MIX},2}}},} & (3)\end{matrix}$

where ideally ΔI_(MIX)=0. Thus, applying a constant T_(MIX) in Equation(1) provides at least a factor of 2 improvement over the case shown inEquation (2). However, this also indicates that I_(MIX (eq. 1)) will notremain constant for changes in sensing frequency f_(SENS), as the sensorelectrode may not be fully settled causing the initial conditions of thesensor electrode to change with different sensing frequencies f_(SENS).

In some embodiments, different T_(MIX) values are applied for differentsensing frequencies f_(SENS). To force ΔI_(MIX)=0, the followingrelationship must be satisfied:

$\begin{matrix}{{T_{{MIX},1} = {T_{{MIX},2} - {2 \cdot R_{RX} \cdot C_{RX} \cdot {\ln\left\lbrack {1 + {\alpha \cdot \left( {1 - e^{- \frac{T_{{MIX},2}}{R_{RX} \cdot {({C_{T} + C_{RX}})}}}} \right)}} \right\rbrack}}}},} & (4) \\{where} & \; \\{{1 + \alpha} = {\frac{1 + {\tanh \left( \frac{1}{4 \cdot f_{{SENS},2} \cdot R_{RX} \cdot \left( {C_{T} + C_{RX}} \right)} \right)}}{1 + {\tanh \left( \frac{1}{4 \cdot f_{{SENS},1} \cdot R_{RX} \cdot \left( {C_{T} + C_{RX}} \right)} \right)}}.}} & (5)\end{matrix}$

Therefore, a desired linear relationship between I_(MIX,1) and I_(MIX,2)can be achieved by applying different T_(MIX) values for differentsensing frequencies f_(SENS). While the analysis provided above is interms of a single-pole model (i.e., assuming that R_(TX)=0 andC_(TX)=0), the performance of an exemplary distributed RC model in whichdifferent T_(MIX) values are applied for different sensing frequenciesf_(SENS) is illustrated further in FIG. 12. Further, in some embodimentsa desired gain value A may be applied to the input current I_(IN)(illustrated in plot 820) via the current conveyor.

FIG. 9 is a schematic diagram of a sensing implementation 900 (or“implementation”) for operating one or more switching elements atdifferent sensing frequencies, according to embodiments describedherein. The implementation 900 is capable of use in conjunction withvarious embodiments discussed herein, such as the implementations 200,300 of sensor electrodes discussed above with respect to FIGS. 2 and 3and the processing system depicted in FIG. 4.

The implementation 900 comprises the voltage source 505 that generates acapacitive sensing signal having a voltage waveform V_(TX). The voltagewaveform V_(TX) may have any suitable shape, and the frequency ofvoltage waveform V_(TX) is controlled based on which predefined sensingfrequency (e.g., f_(SENS,1), f_(SENS,2), f_(SENS,3), . . . , f_(SENS,K)of FIG. 4) is selected. The voltage source 505 drives the capacitivesensing signal onto a sensor electrode 510, which is represented as afirst-order model having a transmitter resistance R_(TX) and atransmitter capacitance C_(TX). Based on a transcapacitance C_(T)between sensor electrode 510 and sensor electrode 515, the sensorelectrode 515 receives resulting signals which are provided to receivercircuitry 905. The sensor electrode 515 is also represented as afirst-order model having a receiver resistance R_(RX) and a receivercapacitance C_(RX). However, the principles discussed herein also applyto more complex (e.g., distributed) modeling of the plurality of sensorelectrodes. Further, although described in terms of a transcapacitiveimplementation having two separate sensor electrodes 510, 515, theprinciples discussed herein also apply to absolute capacitiveimplementations in which the same sensor electrode(s) are used totransmit and receive capacitive sensing signals.

As shown, the receiver circuitry 905 comprises the amplifier 525 (or“op-amp”), the current conveyor (or “current mirror”) 530, and thedemodulator (or “mixer”) 535. The input current I_(IN) represents theresulting signals received by the sensor electrode 515, and is mirroredby the current conveyor 530 (with any suitable gain value A) as anoutput current I_(OUT). Based on a received demodulation signal, thedemodulator 535 downconverts the output current I_(OUT) having higher(RF) frequencies to a demodulated current I_(MIX), which may be filteredby subsequent circuitry to have approximately direct current (DC) levels(i.e., having substantially no frequency component). As shown, a voltagesource 540 generates the demodulation signal with a voltage waveformV_(MIX) that is based on a selected mixing period (e.g., T_(MIX,1),T_(MIX,2), T_(MIX,3), . . . , T_(MIX,K) of FIG. 4).

The demodulator 535 is generally a continuous-time demodulator havingany suitable implementation, such as a square-wave mixer, harmonicrejection mixer, or sinusoidal mixer. In some embodiments, thedemodulator 535 receives a three-level demodulation signal having apositive level, a negative level, and a zero level.

In implementation 900, a first switching element S1 is configured toselectively couple the voltage source 505 and the sensor electrode 510,and a second switching element S2 is configured to selectively couplethe sensor electrode 515 and the receiver circuitry 905. The firstswitching element S1 is controlled by a control signal 915-1, and thesecond switching element S2 is controlled by a control signal 915-2. Insome embodiments, a calculation block 910 outputs a magnitude of thevoltage waveform V_(MIX) as the control signals 915-1, 915-2. Thus, inthe case of a three-level demodulation signal, each of the positivelevel and the negative level corresponds to a conducting state of theswitching elements S1 and S2, and the zero level corresponds to anon-conducting state of the switching elements S1 and S2. In this way,the receiver circuitry 905 operates the switching elements S1, S2 toselectively couple the respective sensor electrodes 510, 515 into thesensing path beginning from the voltage source 505 and ending at theinput to the receiver circuitry 905. Beneficially, the switchingelements S1 and S2 may be implemented within the digital domain withoutrequiring relatively larger analog hardware.

In some embodiments, the switching elements S1, S2 couple the respectivesensor electrodes 510, 515 into the sensing path only during mixingperiods of a sensing period, defined by a selected mixing periodT_(MIX). In such embodiments, the value of mixing period T_(MIX) isconstant for different sensing frequencies f_(SENS). Because switchingelements S1, S2 are closed (conducting) for a fixed amount of time(i.e., the mixing period T_(MIX)) during each sensing half-cycle, chargeis conserved on the sensor electrodes and the amount of input currentI_(IN) substantially remains the same across the different sensingfrequencies f_(SENS). Assuming that R_(TX)=0 and C_(TX)=0, the averagecurrent value during a sensing half-cycle can be represented as:

$\begin{matrix}{I_{MIX} = {\frac{C_{T} \cdot V_{TX}}{T_{MIX}} \cdot {\quad{{\left\lbrack {1 + {e^{- \frac{T_{MIX}}{R_{RX} \cdot {({C_{T} + C_{RX}})}}} \cdot \left( {e^{- \frac{T_{MIX}}{R_{RX} \cdot {({C_{T} + C_{RX}})}}} - 1} \right)}} \right\rbrack \cdot \left( {1 - e^{- \frac{T_{MIX}}{R_{RX} \cdot {({C_{T} + C_{RX}})}}}} \right)},}}}} & (6)\end{matrix}$

illustrating that to a first order, the average current value of I_(MIX)will be independent of the sensing frequency f_(SENS).

While two switching elements S1, S2 are depicted in implementation 900,alternate implementations may include a single switching element withinthe sensing path. Further, while single sensor electrodes 510, 515 areshown as coupled with respective switching elements S1, S2, alternateimplementations may have multiple sensor electrodes (e.g., a pluralityof sensor electrodes within a predefined group) coupled with a singleswitching element.

In some embodiments, the number and location of the one or moreswitching elements are selected based on the time constants of theassociated sensor electrodes 510, 515. As is known to the person ofordinary skill in the art, the time constant associated with aparticular sensor electrode depends upon both the sensor electroderesistance and the sensor electrode capacitance. For example, the timeconstant associated with sensor electrode 510 depends on the values ofR_(TX) and C_(TX). For the case in which the time constant associatedwith the sensor electrode 510 is dominant (i.e., significantly larger)than that of the sensor electrode 515, the implementation 900 operatesswitching element S1 while maintaining an electrical connection betweenthe sensor electrode 515 and the input to the receiver circuitry 905. Insome implementations, the switching element S2 is omitted from theimplementation 900 and replaced by a direct electrical connection. Inother implementations, the control signals 915-1, 915-2 are independentand the control signal 915-2 causes the switching element S2 to remainin a conducting state to form the electrical connection.

Similarly, in the case in which the time constant associated with thesensor electrode 515 is dominant, the implementation 900 may operateswitching element S1 independently of switching element S2 to maintainan electrical connection between the voltage source 505 and the sensorelectrode 510, or may omit switching element S1 entirely. In the case inwhich neither time constant is dominant (e.g., the time constants are onthe same order), both switching elements S1, S2 are included in theimplementation 900 and may be operated using a same control signal forcontrol signals 915-1, 915-2.

FIG. 10 illustrates a method 1000 for operating one or more switchingelements at different sensing frequencies, according to embodimentsdescribed herein. The method 1000 is capable of use in conjunction withvarious embodiments discussed herein, such as the processing systemdepicted in FIG. 4 and the sensing implementation 900 of FIG. 9, or anyother suitable processing system.

Method 1000 begins at block 1005, where the processing system drivesonto a first group of a plurality of sensor electrodes, a firstcapacitive sensing signal having a predefined first sensing frequency.At block 1015, the processing system acquires, based on the driven firstcapacitive sensing signal, first capacitive measurements of resultingsignals received by a second group of the plurality of sensorelectrodes. The processing system applies a first demodulation signalhaving a predefined mixing period defined within a sensing periodassociated with the first sensing frequency. In some embodiments, thepredefined first sensing frequency is selected based on one or moreidentified slowest sensor electrodes. In some cases, the predefinedfirst sensing frequency is one of a plurality of predefined sensingfrequencies, and the predefined mixing period is selected to fit withinsensing half-cycles of the greatest (i.e., highest frequency) of thepredefined sensing frequencies.

At block 1025, the processing system operates, based on the firstdemodulation signal, one or more switching elements coupled with one ormore sensor electrodes of the first group or the second group, whereinthe one or more switching elements are in a conducting state during themixing period. In this way, the processing system operates the one ormore switching elements to selectively couple the respective sensorelectrodes into the sensing path.

At optional block 1035, the processing system drives, onto a third groupof the plurality of sensor electrodes, a second capacitive sensingsignal having a second sensing frequency different from the firstsensing frequency. The third group may be the same as the first group,but this is not a requirement. In some cases, the transition between thefirst sensing frequency and the second sensing frequency is performedresponsive to detected interference at the first sensing frequency.

At optional block 1045, the processing system acquires, based on thedriven second capacitive sensing signal, second capacitive measurementsof resulting signals received by a fourth group of the plurality ofsensor electrodes. The fourth group may be the same as the second group,but this is not a requirement. The processing system applies a seconddemodulation signal having the mixing period. Because the one or moreswitching elements are in a conducting state for a fixed amount of time(i.e., the predefined mixing period) during each sensing half-cycle, theaverage current value for input current to receiver circuitry of theprocessing system remains substantially the same for the first sensingfrequency and the second sensing frequency. Method 1000 ends followingcompletion of block 1045.

FIG. 11 is a diagram 1100 illustrating exemplary operation of a sensingimplementation using one or more switching elements, according toembodiments described herein. More specifically, diagram 1100illustrates exemplary operation of the sensing implementation 900 ofFIG. 9, or any other suitable sensing implementation.

Diagram 1100 includes a plot 1105 depicting a three-level demodulationsignal (V_(MIX)) corresponding to a sensing frequency of 200 kHz. Plot1110 depicts an input current I_(IN) resulting from driving one or moresensor electrodes at the 200 kHz sensing frequency. Within a sensingcycle 1120-1, the positive level (i.e., 1V) of the demodulation signalis applied during a positive sensing half-cycle 1112-1, and the negativelevel (i.e., −1V) of the demodulation signal is applied during anegative sensing half-cycle 1116-1. Each of the positive sensinghalf-cycle 1112-1 and the negative sensing half-cycle 1116-1 correspondto a predefined mixing period T_(MIX). The sensing cycle 1120-1 alsoincludes predefined periods 1114-1, 1118-1 (or “non-mixing periods”),during which the zero level of the demodulation signal is applied. Asshown, the predefined periods 1114-1, 1118-1 have a substantially equallength, but this is not a requirement. According to embodimentsdiscussed herein, switching element(s) that are coupled with the sensorelectrode(s) are in a conducting state during each mixing periodT_(MIX), and are in a non-conducting state during each non-mixing period1114-1, 1118-1.

Plot 1125 depicts the three-level demodulation signal V_(MIX)corresponding to a sensing frequency of 250 kHz, and plot 1130 depictsthe corresponding input current I_(IN) at the sensing frequency.Similarly, plot 1135 depicts the three-level demodulation signal V_(MIX)corresponding to a sensing frequency of 300 kHz, and plot 1140 depictsthe corresponding input current I_(IN). Within each of the sensingcycles 1120-2 (250 kHz), 1120-3 (300 kHz), the positive sensinghalf-cycles 1112-2, 1112-3 and negative sensing half-cycles 1116-2,1116-3 each correspond to the predefined mixing period T_(MIX), whilethe predefined periods 1114-2, 1118-2, 1114-3, 1118-3 are shortened assensing frequency increases. Because the switching element(s) are in aconducting state for a fixed amount of time (i.e., the predefined mixingperiod T_(MIX)) during each sensing half-cycle, the average currentvalue for the input current I_(IN) remains substantially the same acrossthe different sensing frequencies. And because the average current valueis controlled to be substantially the same for different sensingfrequencies, any shift occurring in the capacitive baseline whentransitioning between different sensing frequencies is partly or fullymitigated.

FIG. 12 is a graph 1200 illustrating changes in capacitance acrossdifferent sensing frequencies, according to embodiments describedherein. More specifically, graph 1200 illustrates simulation results fora distributed RC model of the plurality of sensor electrodes, andcorresponding to a transcapacitance C_(T) of 6.4 picofarads (pF). Withingraph 1200, a plot 1205 represents a case in which the predefined mixingperiod equals half of the period corresponding to the particular sensingfrequency (similar to FIG. 7). Plot 1205 illustrates a substantiallylinear change in baseline capacitance over the depicted range of sensingfrequencies (160 kHz to 195 kHz), which will be compensated by theprocessing system when transitioning between different sensingfrequencies.

Plot 1210 represents an implementation having a constant mixing periodapplied across the range of sensing frequencies. As shown, andconsistent with the discussion of Equation (3) above, applying aconstant mixing period offers at least a factor of 2 improvement (i.e.,reduction or mitigation) of the baseline capacitance shift, whencompared with plot 1205. Plot 1215 represents an implementation havingswitching elements selectively coupling sensor electrodes during mixingperiods (such as implementation 900 of FIG. 9), which provides anadditional reduction or mitigation of the baseline capacitance shift.Plot 1220 represents an implementation having a variable mixing periodthat depends on the selected sensing frequency (similar to FIG. 8). Amaximum value of the plot 1220 is approximately 5 femtofarads (fF),offering an additional reduction or mitigation of the baselinecapacitance shift.

Thus, the embodiments and examples set forth herein were presented inorder to best explain the embodiments in accordance with the presenttechnology and its particular application and to thereby enable thoseskilled in the art to make and use the disclosure. However, thoseskilled in the art will recognize that the foregoing description andexamples have been presented for the purposes of illustration andexample only. The description as set forth is not intended to beexhaustive or to limit the disclosure to the precise form disclosed.

In view of the foregoing, the scope of the present disclosure isdetermined by the claims that follow.

What is claimed is:
 1. A method comprising: acquiring, responsive to afirst capacitive sensing signal driven on a first group of one or moresensor electrodes, first capacitive measurements of resulting signalsreceived by a second group of one or more sensor electrodes, whereinacquiring the first capacitive measurements comprises applying a firstdemodulation signal having a predefined first mixing period, the firstcapacitive sensing signal having a first sensing frequency; andacquiring, responsive to a second capacitive sensing signal driven on athird group of one or more sensor electrodes, second capacitivemeasurements of resulting signals received by a fourth group of one ormore sensor electrodes, wherein acquiring the second capacitivemeasurements comprises applying a second demodulation signal having apredefined second mixing period, the second capacitive sensing signalhaving a second sensing frequency, wherein the second mixing period isdifferent than the first mixing period and is selected such that asecond average current value for the second capacitive measurements hasa predefined relationship to a first average current value for the firstcapacitive measurements, the predefined relationship based on the firstsensing frequency and the second sensing frequency.
 2. The method ofclaim 1, wherein according to the predefined relationship, the secondaverage current value is substantially equal to the first averagecurrent value.
 3. The method of claim 1, wherein according to thepredefined relationship, the second average current value has asubstantially linear relation to the first average current value.
 4. Themethod of claim 1, wherein each sensing period comprises a positivesensing half-cycle and a negative sensing half-cycle, wherein thepositive sensing half-cycle and the negative sensing half-cycle eachhave a length corresponding to the first mixing period or to the secondmixing period.
 5. The method of claim 4, wherein the first demodulationsignal and the second demodulation signal are three-level demodulationsignals having a positive level, a negative level, and a zero level,wherein applying the first demodulation signal and applying the seconddemodulation signal each comprises: applying the positive level duringthe positive sensing half-cycle; applying the negative level during thenegative sensing half-cycle; and applying the zero level during apredefined period within the sensing period between the positive sensinghalf-cycle and the negative sensing half-cycle.
 6. The method of claim1, wherein the first sensing frequency and the second sensing frequencyare included in a predefined plurality of sensing frequencies, whereinthe first mixing period and the second mixing period are included in apredefined plurality of mixing periods selected based on an identifiedone or more slowest sensor electrodes.
 7. The method of claim 6, whereinthe predefined plurality of mixing periods are further selected based onone or more fastest sensing frequencies of the predefined plurality ofsensing frequencies.
 8. A processing system comprising: a sensing modulecomprising sensing circuitry and configured to: acquire, responsive to afirst capacitive sensing signal driven on a first group of one or moresensor electrodes, first capacitive measurements of resulting signalsreceived by a second group of one or more sensor electrodes, whereinacquiring the first capacitive measurements comprises applying a firstdemodulation signal having a predefined first mixing period, the firstcapacitive sensing signal having a first sensing frequency; and acquire,responsive to a second capacitive sensing signal driven on a third groupof one or more sensor electrodes, second capacitive measurements ofresulting signals received by a fourth group of one or more sensorelectrodes, wherein acquiring the second capacitive measurementscomprises applying a second demodulation signal having a predefinedsecond mixing period, the second capacitive sensing signal having asecond sensing frequency, wherein the second mixing period is differentthan the first mixing period and is selected such that a second averagecurrent value for the second capacitive measurements has a predefinedrelationship to a first average current value for the first capacitivemeasurements, the predefined relationship based on the first sensingfrequency and the second sensing frequency.
 9. The processing system ofclaim 8, wherein according to the predefined relationship, the secondaverage current value is substantially equal to the first averagecurrent value.
 10. The processing system of claim 8, wherein accordingto the predefined relationship, the second average current value has asubstantially linear relation to the first average current value. 11.The processing system of claim 8, wherein each sensing period comprisesa positive sensing half-cycle and a negative sensing half-cycle, whereinthe positive sensing half-cycle and the negative sensing half-cycle eachhave a length corresponding to the first mixing period or to the secondmixing period.
 12. The processing system of claim 11, wherein the firstdemodulation signal and the second demodulation signal are three-leveldemodulation signals having a positive level, a negative level, and azero level, wherein applying the first demodulation signal and applyingthe second demodulation signal each comprises: applying the positivelevel during the positive sensing half-cycle; applying the negativelevel during the negative sensing half-cycle; and applying the zerolevel during a predefined period within the sensing period between thepositive sensing half-cycle and the negative sensing half-cycle.
 13. Theprocessing system of claim 8, wherein the first sensing frequency andthe second sensing frequency are included in a predefined plurality ofsensing frequencies, wherein the first mixing period and the secondmixing period are included in a predefined plurality of mixing periodsselected based on an identified one or more slowest sensor electrodes.14. The processing system of claim 13, wherein the predefined pluralityof mixing periods are further selected based on one or more fastestsensing frequencies of the predefined plurality of sensing frequencies.15. An input device comprising: a plurality of sensor electrodes; and aprocessing system coupled with the plurality of sensor electrodes,wherein the processing system is configured to: acquire, responsive to afirst capacitive sensing signal driven on a first group of one or moresensor electrodes, first capacitive measurements of resulting signalsreceived by a second group of one or more sensor electrodes, whereinacquiring the first capacitive measurements comprises applying a firstdemodulation signal having a predefined first mixing period, the firstcapacitive sensing signal having a first sensing frequency; acquire,responsive to a second capacitive sensing signal driven onto a thirdgroup of one or more sensor electrodes, second capacitive measurementsof resulting signals received by a fourth group of one or more sensorelectrodes, wherein acquiring the second capacitive measurementscomprises applying a second demodulation signal having a predefinedsecond mixing period, the second capacitive sensing signal having asecond sensing frequency, wherein the second mixing period is differentthan the first mixing period and is selected such that a second averagecurrent value for the second capacitive measurements has a predefinedrelationship to a first average current value for the first capacitivemeasurements, the predefined relationship based on the first sensingfrequency and the second sensing frequency.
 16. The input device ofclaim 15, wherein according to the predefined relationship, the secondaverage current value is substantially equal to the first averagecurrent value.
 17. The input device of claim 15, wherein according tothe predefined relationship, the second average current value has asubstantially linear relation to the first average current value. 18.The input device of claim 15, wherein each sensing period comprises apositive sensing half-cycle and a negative sensing half-cycle, whereinthe positive sensing half-cycle and the negative sensing half-cycle eachhave a length corresponding to the first mixing period or to the secondmixing period.
 19. The input device of claim 18, wherein the firstdemodulation signal and the second demodulation signal are three-leveldemodulation signals having a positive level, a negative level, and azero level, wherein applying the first demodulation signal and applyingthe second demodulation signal each comprises: applying the positivelevel during the positive sensing half-cycle; applying the negativelevel during the negative sensing half-cycle; and applying the zerolevel during a predefined period within the sensing period between thepositive sensing half-cycle and the negative sensing half-cycle.
 20. Theinput device of claim 15, wherein the first sensing frequency and thesecond sensing frequency are included in a predefined plurality ofsensing frequencies, wherein the first mixing period and the secondmixing period are included in a predefined plurality of mixing periodsselected based on an identified one or more slowest sensor electrodes ofthe plurality of sensor electrodes.